1. Field of the Invention
The invention relates to apparatus, and an accompanying method for use therein, that utilizes first (working) and second (stopping) servo-controlled hydraulic pistons wherein the second piston acts as a controlled mechanical stop for the first piston. Advantageously, the apparatus can controllably stop the first piston, traveling at relatively high speeds, in a very short time and over a very short distance while advantageously inducing very little, if any, elastic strain into the apparatus.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products. Such materials are produced
typically in large ingots or other shapes and are controllably deformed by, e.g., rolling, forging or extruding into readily useable and conventional sheet, plate, coil or wire form for subsequent machining or forming. These deformations typically occur on a repeated incremental basis, such as through a multi-stand rolling mill where the material is repeatedly passed through successive pairs of rolls. Each pass incrementally compresses, i.e., deforms, the material into thinner stock. Typically, each pair of rolls is spaced equal distance from the next pair but has a smaller inter-roll spacing (xe2x80x9croll bitexe2x80x9d) than the next pair. Hence, as the material becomes thinner it travels at a faster rate through successive roll pairs and this decreases the time occurring between each compression. Extrusion, forging and braking operations also typically involve incremental deformations until the material is properly sized.
In production environments, small incremental deformations are typically produced at high rates. However, correctly configuring a mill, forge or brake to properly deform production stock and impart a desired amount of strain to the material along with other physical/metallurgical characteristics can be a tedious, time-consuming and expensive processxe2x80x94particularly since such a machine needs to be taken out of productive use for an extended time to properly adjust its operational parameters. Consequently, to avoid the need for costly downtime, thermodynamic material testing systems are employed to simulate rolling, extruding, braking and forging processes on relatively small metallic specimens. Resulting simulation data is then used to properly set various operating parameters of production equipment and, by doing so, minimize its non-productive downtime. Illustrative simulators of this type include the xe2x80x9cGleeblexe2x80x9d and xe2x80x9cHydrawedgexe2x80x9d systems manufactured by Dynamic Systems Inc. (DSI) of Poestenkill, N.Y., which is the present assignee hereof (with xe2x80x9cGleeblexe2x80x9d and xe2x80x9cHydrawedgexe2x80x9d being registered trademarks owned by DSI).
Systems which deform metallic materials, particularly including material testing systems, often utilize linear motion of a piston/anvil combination produced by servo-controlled hydraulic systems, and particularly those which accelerate and stop pistons at very high speeds. Such movement is necessary to impart a desired amount of deformation to the material, situated, e.g., between a pair of anvils, at a desired strain rate and over as much of a resulting deformation as possible. In these environments, linear piston systems moving with velocities up to 10 meters per second are frequently used, with velocities of 1 to 2 meters per second being quite common.
In particular, in such testing systems, a fundamental problem arises in that a piston, while traveling at such a high relative velocity, must often be stopped in a manner, essentially immediately, through which its velocity does not decrease even over a small distance, else the strain rate imparted to the specimen will decrease over a stopping distance of the piston. Further, those systems typically utilize mechanical mechanisms of one sort or another to stop the piston which, while the piston is being stopped, disadvantageously introduce some strain into various structural components of the system itself. This added strain, by effectively compressing a frame of the system, tends to slightly elongate the stopping distance and thus adversely impact the resulting deformation of the specimen.
Another area in which high-speed deformation is becoming increasingly important is sheet metal processing. Here, a need to reduce production costs requires that press brakes used to deform metallic material, i.e., bend metal sheets, operate at increasingly high speeds. Conventional bending machines have a set of shaped dies in which the material is held and then formed or bent. The dies are mounted in rather large, heavy beam structures. Usually, one beam is mounted rigidly, while another is mounted on linear sliding ways. Traditional brakes rely on producing linear motion for the ways through a large flywheel and suitable connecting/pivoting arms mounted between one beam and the flywheel. Relatively modern brakes control the motion of the die/beam using hydraulic servo-controlled piston/cylinder systems. A precision with which the material can be bent depends upon how quickly the die can be stopped at a bottom of its stroke (travel). As the speed of the die increases, its stopping distance becomes increasingly arbitrary. Given this, the stroke often has to be disadvantageously run at a decreased speed to consistently stop the die at a precise location. Additionally, the metal being deformed often provides a variable load to the die. This variable load causes the control system to compensate while the dies are being stopped at a desired position, but again generally necessitates that reduced speeds are used to obtain precise bends in pieces then being processed. Such decreased speeds disadvantageously reduce material throughput. Hence, to consistently deform material at relatively high speeds and increase throughput, the stroke has to be accurately controlled both in terms of its velocity, throughout the stroke length, as well as its stopping distance.
In situations, be it in material testing systems or in production equipment, where material is being deformed at high-speeds, mechanical stops are often used to stop a high-speed anvil, ram or die at a precise position. Unfortunately, a position of such a mechanical stop has to be changed each time the desired amount of travel is changed.
Therefore, apparatus is needed to stop motion of, e.g., an anvil, a ram or die in an exact position even at very high speeds in order to provide consistent results. Such a stop should impart very little, if any, strain in the apparatus so that the stopping position remains the same regardless of the changes in a load then being deformed. This entails that an end of a high-speed stroke must be precisely controlled as well as being easily and rapidly changeable.
U.S. Pat. No. 5,092,179 (issued to H. S. Ferguson on Mar. 3, 1992) describes one such thermodynamic material testing system. As shown in FIG. 5 thereof, a stroke of piston 509 and shafts 540 and 545 are stopped by stop disc 543. A position of specimen material 570, being deformed, is advanced by hydraulic cylinder 590, piston 592, wedge combination 585/582, shaft 575, load cell 574, plate 568, anvil base 565 and anvil 560xe2x80x2. Each time specimen 570 is advanced by the wedge combination toward the left, anvil 560 is retracted and then rapidly advanced to the right, thus deforming specimen 570 until stop plate 543 hits cross-stop 550. A drawback inherent in this system is that, during each hit, an amount of strain occurs elastically in an entire wedge assembly that supports a load on the right side of the specimen (load cell side). This elastic strain allows anvil 560 to move in the direction of deformation, thus decreasing the amount of deformation in the specimen and slightly compromising a final thickness of the specimen after each deformation. Once the system has been used to deform a particular specimen at a certain temperature, a computer-controlled deformation schedule (deformation program) that controls the system can be modified to accommodate for expected loss (increased material thickness) in deformation that would result from the elastic strain. However, doing so is a passive correction and never exact. For multiple deformations involving 3 or more hits to the specimen, appropriate modifications to the program become time-consuming and tedious to determine. Further, each hit becomes less precise as the number of hits increases. Therefore, a stopping mechanism is readily desired, for use in a thermodynamic material testing system, that imparts very little, if any, strain back into any structural component of the system itself during each hit.
Thus, the need still exists in the art for a stopping mechanism for use with a servo-controlled hydraulic system, such as that used in a material testing system, in which a piston can be stopped from a very high speed at an exact location without over-travel and without substantial reduction in its speed right up to the moment of its stopping. The system should be capable of repetitive hits with each final stop at a predetermined position regardless of the speed of the piston. The stopping system should produce very little, if any, strain.
Further evidence for the need for such a stopping mechanism can be seen from the following. A modern high-speed servo valve can be closed, from 80 percent of its maximum opening, typically in 0.003 seconds. If a piston controlled by that valve is moving at 1 meter per secondxe2x80x94which often occurs in production equipment and material testing systems, then the piston will travel approximately 1.5 mm during stopping. This distance is clearly unacceptable where, in testing systems and high-speed press brakes, distances controlled to less than 0.05 mm are desired. Linkages, shafts and wedges of existing stopping mechanisms can have strains, under expected operating loads, of 0.3 or 0.4 mm. While these reduced strains are considerably better than that which results from use of no stopping mechanism at all, a stopping mechanism that produces far less strain in the mechanism itself is still needed.
The present invention advantageously overcomes the deficiencies associated with high-speed use of servo-controlled hydraulic systems known in the art where very rapid stopping is required with essentially little, if any, strain occurring in the stopping mechanism. Through the invention, a first high-speed (working) piston is stopped by an adjustable mechanical stop, formed of a second (stopping) piston coaxially situated to the working piston.
Advantageously, the present invention permits the stopping position of the working piston to be rapidly changed. Furthermore, the invention, by virtue of its stopping characteristics and inducing minimal resulting strain in the stopping mechanism, permits the servo system to repetitively and rapidly actuate the working piston, many times, with nearly ideal stopping positions each time regardless of the speed of that piston. High-speed stopping occurs over extremely short stopping distances and is essentially immediate.
In accordance with the teachings of the invention, the working and stopping pistons are controlled by separate servo-control hydraulic systems and are both coaxially located on a common piston shaft for the working piston, with these pistons being longitudinally spaced apart on the shaft. Both pistons controllably move within separate corresponding piston cylinders. The stopping piston slides on the piston shaft with the shaft extending through a central longitudinally-oriented bore on the stopping piston. To stop further movement of the working piston, the stopping piston abuttingly engages, via complementary surfaces, with a radially extending circular stopping element on the shaft, e.g., a shoulder extending outward from and concentric with the shaft. Preferably and to provide positive stopping action, the stopping piston is sized and operated with sufficient hydraulic pressure to provide higher forces than the working piston. Illustratively, the working piston may provide a maximum force of 40 tons and the stopping piston a maximum force of 80 tons or more.
In operation, the stopping piston is programmably moved, through appropriate computer-control of its servo-control hydraulic system, to a desired stopping position for the working piston. The working piston is retracted (before, coincident with or after the stopping piston is moved) and, once the stopping piston is properly position, then extended at a high speed. The working piston stops its extension whenever a surface of the stopping element on the piston shaft abuttingly engages a complementary surface situated on an upper side of the stopping piston. To change the stopping position, the stopping piston is simply moved, again through appropriate control over the servo-controlled hydraulic system, and the process is repeated, and so forth for multiple hits.
The stopping mechanism is comprised of only the stopping piston and hydraulic oil used to position that piston. There are no stopping linkages, wedges, shafts or other mechanical parts which would change dimension under a changing load. Accordingly, the amount of strain that occurs in the stopping mechanism is significantly reduced.
In accordance with a feature of the invention, two stopping elements (i.e., upper and lower stopping elements) can be positioned on the piston shaft, with a corresponding stopping element situated on either side of the stopping piston. In this manner, and with complementary surfaces being formed on the upper and lower surfaces of that piston, the stopping piston can stop movement of the working piston in both its upward and downward (retraction and extension) directions, rather than just in a downward direction.